_wp_post_thumbnail_html

function _wp_post_thumbnail_html( $thumbnail_id = null, $post = null ) {
	$_wp_additional_image_sizes = wp_get_additional_image_sizes();

	$post               = get_post( $post );
	$post_type_object   = get_post_type_object( $post->post_type );
	$set_thumbnail_link = '<p class="hide-if-no-js"><a href="%s" id="set-post-thumbnail"%s class="thickbox">%s</a></p>';
	$upload_iframe_src  = get_upload_iframe_src( 'image', $post->ID );

	$content = sprintf(
		$set_thumbnail_link,
		esc_url( $upload_iframe_src ),
		'', // Empty when there's no featured image set, `aria-describedby` attribute otherwise.
		esc_html( $post_type_object->labels->set_featured_image )
	);

	if ( $thumbnail_id && get_post( $thumbnail_id ) ) {
		$size = isset( $_wp_additional_image_sizes['post-thumbnail'] ) ? 'post-thumbnail' : array( 266, 266 );

		/**
		 * Filters the size used to display the post thumbnail image in the 'Featured image' meta box.
		 *
		 * Note: When a theme adds 'post-thumbnail' support, a special 'post-thumbnail'
		 * image size is registered, which differs from the 'thumbnail' image size
		 * managed via the Settings > Media screen. See the `$size` parameter description
		 * for more information on default values.
		 *
		 * @since 4.4.0
		 *
		 * @param string|array $size         Post thumbnail image size to display in the meta box. Accepts any valid
		 *                                   image size, or an array of width and height values in pixels (in that order).
		 *                                   If the 'post-thumbnail' size is set, default is 'post-thumbnail'. Otherwise,
		 *                                   default is an array with 266 as both the height and width values.
		 * @param int          $thumbnail_id Post thumbnail attachment ID.
		 * @param WP_Post      $post         The post object associated with the thumbnail.
		 */
		$size = apply_filters( 'admin_post_thumbnail_size', $size, $thumbnail_id, $post );

		$thumbnail_html = wp_get_attachment_image( $thumbnail_id, $size );

		if ( ! empty( $thumbnail_html ) ) {
			$content  = sprintf(
				$set_thumbnail_link,
				esc_url( $upload_iframe_src ),
				' aria-describedby="set-post-thumbnail-desc"',
				$thumbnail_html
			);
			$content .= '<p class="hide-if-no-js howto" id="set-post-thumbnail-desc">' . __( 'Click the image to edit or update' ) . '</p>';
			$content .= '<p class="hide-if-no-js"><a href="#" id="remove-post-thumbnail">' . esc_html( $post_type_object->labels->remove_featured_image ) . '</a></p>';
		}
	}

	$content .= '<input type="hidden" id="_thumbnail_id" name="_thumbnail_id" value="' . esc_attr( $thumbnail_id ? $thumbnail_id : '-1' ) . '" />';

	/**
	 * Filters the admin post thumbnail HTML markup to return.
	 *
	 * @since 2.9.0
	 * @since 3.5.0 Added the `$post_id` parameter.
	 * @since 4.6.0 Added the `$thumbnail_id` parameter.
	 *
	 * @param string   $content      Admin post thumbnail HTML markup.
	 * @param int      $post_id      Post ID.
	 * @param int|null $thumbnail_id Thumbnail attachment ID, or null if there isn't one.
	 */
	return apply_filters( 'admin_post_thumbnail_html', $content, $post->ID, $thumbnail_id );
}